The present invention relates to the manufacture of glass or ceramic products from particulate oxides, and particularly to an improved method of making such products by the direct casting of non-aqueous suspensions.
It has been recognized that very pure glass and ceramic products can be made using pure fumed oxide starting materials. For example pure fused silica glasses have been used to make glass optical waveguide fibers which exhibit extremely high transparency.
A well known method for producing massive articles from pure fumed oxides is to deposit the oxide particles immediately after they are formed (e.g., in a flame) on a substrate or preform where they adhere to form a porous monolith. This porous body can then be sintered to clear glass and, if necessary, further shaped to a limited degree to provide a product of a desired shape.
There are some shapes which cannot conveniently be formed by the direct deposition of the oxides onto a preform. Accordingly, efforts have been made to develop other ways to process these oxides. However, fumed oxides are generally fluffy, high-surface-area materials (surface areas typically range from 25-400 m.sup.2 /gram, with average particle sizes typically in the range from 0.01-0.5 microns), and they are very difficult to process by slip-casting or other methods conventionally used for shaping ceramic starting materials.
U.S. Pat. Nos. 4,042,361 and 4,200,445 disclose one approach to the problem which involves dispersing the fumed oxides in water to form a suspension. This suspension is then processed by casting into thin sheets, drying to cause fragmentation or dicing into porous granules, sintering the porous granules to form a densified granular material, milling the densified material to form a casting slip, and finally casting the slip to form a green ceramic shape which can be fired to provide a glass product.
The process disclosed in the above two patents disadvantageously requires the handling of aqueous suspensions of fumed oxides which do not exhibit long shelf life and are relatively viscous even at moderate oxide concentrations. In addition, massive articles are not readily cast directly from aqueous suspensions, due to cracking problems. Thus, according to the above patents, a relatively large number of process steps are required before the final cast shape is obtained.
It has recently been proposed, in my co-pending, commonly assigned patent application Ser. No. 492,890, filed May 9, 1983 and expressly incorporated herein by reference, to use non-aqueous dispersions of particulate oxidic materials such as fumed oxides to form glass articles by direct casting into final shape. In this process the cast shape is formed not by slip casting but by gelation of the suspension in its final configuration, followed by removal of the vehicle and sintering of the resulting porous monolith. This method avoids the intermediate dicing and calcining steps required with water suspensions, and the non-aqueous oxide suspensions used are generally stable and easy to cast.
In the process of the above-described patent application the suspensions are generally stable against gelation and accordingly added gelling agents are used to cause rapid conversion of the fluid suspension to a semi-rigid gelled shape. However, there are some applications for which such rapid gelation is not required, and wherein gelling intervals on the order of 24-28 hours would be adequate. Further, applications exist wherein the use of added gelling agents would be an unnecessary or even an undesirable expedient, from either a processing or chemical composition standpoint.